Code Division Multiple Access (CDMA) communication systems may be used to provide wireless communication between a base station and one or more subscriber units. The base station is typically a computer controlled set of switching transceivers that are interconnected to a land-based public switched telephone network (PSTN). The base station includes an antenna apparatus for sending forward link radio frequency signals to the mobile subscriber units. The base station antenna is also responsible for receiving reverse link radio frequency signals transmitted from each mobile unit. Each mobile subscriber unit also contains an antenna apparatus for the reception of the forward link signals and for transmission of the reverse link signals. A typical mobile subscriber unit is a digital cellular telephone handset or a personal computer coupled to a wireless cellular modem.
The most common type of antenna used to transmit and receive signals at a mobile subscriber unit is a omni-directional monopole antenna. This type of antenna consists of a single wire or antenna element that is coupled to a transceiver within the subscriber unit. The transceiver receives reverse link signals to be transmitted from circuitry within the subscriber unit and modulates the signals onto the antenna element at a specified frequency assigned to that subscriber unit. Forward link signals received by the antenna element at a specified frequency are demodulated by the transceiver and supplied to processing circuitry within the subscriber unit. In CDMA cellular systems, multiple mobile subscriber units may transmit and receive signals on the same frequency and use coding algorithms to detect signaling information intended for individual subscriber units on a per unit basis.
The transmitted signal sent from a monopole antenna is omnidirectional in nature. That is, the signal is sent with the same signal strength in all directions in a generally horizontal plane. Reception of signals with a monopole antenna element is likewise omnidirectional. A monopole antenna does not differentiate in its ability to detect a signal on one direction versus detection of the same or a different signal coming from another direction.
Various problems are inherent in prior art antennas used on mobile subscriber units in wireless communications systems. Typically, an antenna array with scanning capabilities consists of a number of antenna elements located on top of a ground plane. For the subscriber unit to satisfy portability requirements, the ground plane must be physically small. For example, in cellular communication applications, the ground plane is typically smaller than the wavelength of the transmitted and received signals. Because of the interaction between the small ground plane and the antenna elements, which are typically monopole elements, the peak strength of the beam formed by the array is elevated above the horizon, for example, by about 30xc2x0, even though the beam itself is directed along the horizon. Correspondingly the strength of the beam along the horizon is about 3 db less than the peak strength. Generally, the subscriber units are located at large distances from the base stations such that the angle of incidence between the subscriber unit and the base station is approximately zero. The ground plane would have to be significantly larger than the wavelength of the transmitted/received signals to be able to bring the peak beam down towards the horizon. For example, in an 800 Mhz system, the ground plane would have to be significantly larger than 14 inches in diameter,and in a PCS system operating at about 1900 Mhz, the ground plane would have to be significantly larger than about 6.5 inches in diameter. Ground planes with such large sizes would prohibit using the subscriber unit as a portable device. Since the antenna array is intended for portable applications, a small size array, for example, an array having a reduced height is highly desirable. Further, it is desirable to produce antenna elements with these beam directing features using low-cost mass production techniques.
The present invention greatly reduces problems encountered by the aforementioned prior art antenna systems. The present invention provides an inexpensive low profile dipole antenna for use with a mobile subscriber unit in a wireless same frequency network communications system, such as CDMA cellular communication networks. The antenna is fabricated with printed circuit board (PCB) photo-etching techniques for precise control of the printed structure. The design includes a two solid conductive patches with a very short feed line so that its conductive loss is minimal.
In one aspect of the invention, the dipole antenna includes a planar substrate made of dielectric material. A conductive planar element is layered on one side of the substrate, and a conductive planar ground patch is layered on the other side of the substrate. The conductive planar element is located in an upper region of the substrate, while the conductive planar ground patch is offset from the conductive planar element in lower region of the substrate. That is, the conductive planar element is stacked above the conductive planar ground patch. A feed strip is connected to the bottom of the conductive planar element, and extends from the element to a bottom edge of the substrate and terminates at a bottom feed point. Typically, the feed point is connected to a transmission line for transmitting signals to and receiving signals from the dipole antenna. The conductive planar ground patch includes a bottom end for connecting the ground patch to a ground plane upon which the dipole antenna is mounted. The ground plane is aligned orthonormal to the antenna.
Capacitive coupling between the conductive planar element and the conductive ground patch creates a junction which provides an upper dipole feed point in a midregion of the substrate such that the conductive planar element acts as one element of an unbalanced dipole antenna and the conductive planar ground patch acts as the other element of the unbalanced dipole antenna. The unbalanced dipole antenna forms a beam which may be positionally directed along a horizon that is substantially parallel to the ground plane.
Embodiments of this aspect can include one or more of the following features. The bottom edge of the conductive planar element and the top edge of the conductive planar ground patch define a gap. The width of the gap can be varied to alter the capacitance of the dipole antenna. The feed strip includes an upward extension positioned between a pair of notches at the bottom edge of the conductive planar element, and the conductive planar ground patch includes a snub located in the middle of the top edge of the ground patch. As such, the interaction between the upward extension, the pair of notches, and the snub provides a short-circuited coplanar waveguide. The length and/or the width of the notches can be varied to alter the inductance of the dipole antenna.
Typically, the conductive planar element and the conductive planar ground patch have rectangular shapes with the shorter sides of the element and the ground patch being aligned perpendicular to the bottom edge of the substrate. The width of the conductive planar element and the width of the conductive planar ground patch are sufficient to provide broadband performance. Even though the antenna acts as a half-wave dipole antenna, the height of the conductive planar element and the height of the conductive planar ground patch are reduced to at least a one-sixth wavelength.
The dielectric substrate can be made from, for example, PCB materials such as polystyrene or Teflon. The conductive planar element, the feed strip, and the conductive planar ground patch are usually made from copper.
In a certain embodiment of this aspect, the conductive planar element is connected to a phase shifter. The phase shifter is independently adjustable to affect the phase of a respective signal transmitted from the dipole antenna. Alternatively, the planar element is connected to a delay line. The antenna can be connected to variable or lumped impedance element and/or a switch. Ideally, the peak strength of the directed beam rises no more than about 10xc2x0 above the horizon.